1. Field of the Invention
This invention relates to an optical deflecting apparatus for generating surface acoustic waves in an optical waveguide and deflecting an optical wave or optical waves guided through the optical waveguide by diffracting actions of the surface acoustic waves. This invention particularly relates to an optical deflecting apparatus wherein a wide deflection angle range is obtained.
2. Description of the Prior Art
As disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-183626, there has heretofore been known an optical deflecting apparatus wherein light is made to enter an optical waveguide formed of a material allowing propagation of a surface acoustic wave therethrough, a surface acoustic wave is generated in a direction intersecting the guided optical wave advancing inside of the optical waveguide to effect Bragg diffraction of the guided optical wave by the surface acoustic wave, and the frequency of the surface acoustic wave is continuously changed to continuously change the angle of diffraction (deflection angle) of the guided optical wave. The optical deflecting apparatus of this type is advantageous in that the apparatus can be fabricated small and light and has high reliability because of the absence of mechanical operating elements as compared with a mechanical type optical deflector such as a galvanometer mirror or a polygon mirror, and an optical deflector using an optical deflecting device such as an electro-optic deflector (EOD) or an acousto-optic deflector (AOD).
However, the aforesaid optical deflecting apparatus has the drawback that the deflection angle cannot be adjusted to be large. Specifically, with the optical deflecting apparatus using the optical waveguide, the optical deflection angle is approximately proportional to the frequency of the surface acoustic wave, and therefore the frequency of the surface acoustic wave must be changed up to a very large value in order to obtain a large deflection angle. Thus it is necessary to change the frequency of the surface acoustic wave over a wide band. Also, in order to satisfy the Bragg condition, it is necessary to control the angle of incidence of the guided optical wave upon the surface acoustic wave by continuously changing (steering) the direction of advance of the surface acoustic wave.
In order to satisfy the aforesaid requirements, as disclosed in, for example, the aforesaid Japanese Unexamined Patent Publication No. 61(1986)-183626, there has heretofore been proposed an optical deflecting apparatus wherein a plurality of interdigital transducers (hereinafter abbreviated as IDT) generating surface acoustic waves, the frequency of which changes continuously in frequency bands different from one another, are disposed so that the directions of generation of the surface acoustic waves are different from one another, and the respective IDTs are operated through switching.
However, the proposed optical deflecting apparatus having the aforesaid configuration has the drawback that the diffraction efficiency decreases around the cross-over frequency of the surface acoustic waves generated by the respective IDTs, and therefore the optical amount of the deflected optical wave fluctuates in accordance with the deflection angle.
Also, with the aforesaid configuration, the IDT which bears the portion of a large deflection angle must ultimately be constituted to be able to generate the surface acoustic wave of a very high frequency. This problem will be described below. The deflection angle o of the guided optical wave caused by the acousto-optic interaction between the surface acoustic wave and the guided optical wave is expressed as .alpha.=2.theta. wherein .theta. denotes the angle of incidence of the guided optical wave with respect to the direction of advance of the surface acoustic wave. Also, the formula ##EQU1## applies wherein .lambda. and Ne respectively denote the wavelength and the effective refractive index of the guided optical wave, and .LAMBDA., f and v respectively denote the wavelength, the frequency and the velocity of the surface acoustic wave. Therefore, a deflection angle range .DELTA.(2.theta.) is expressed as EQU .DELTA.(2.theta.)=.DELTA.f.multidot..lambda./Ne.multidot.v.
For example, in order to obtain a deflection angle range .DELTA.(2.theta.) equal to 10.degree. in the case where .lambda.=0.78 .mu.m, Ne=2.2 and v=3,500 m/s, it is necessary that the frequency range .DELTA.f of the surface acoustic wave, i.e. the frequency band of the high frequency applied to the IDT, be .DELTA.f=1.72 GHz. In the case where said frequency band is of one octave so that adverse effects of the second order diffracted optical wave component can be avoided, the center frequency f0 is equal to 2.57 GHz and the maximum frequency f2 is equal to 3.43 GHz. The period .LAMBDA. of the IDT that gives said maximum frequency f2 is equal to 1.02 .mu.m, and the line width W of the IDT finger is equal to .LAMBDA./4=0.255 .mu.m.
With the current photolithography and electron beam drawing processes which are popular techniques for forming the IDT, the possible line widths are limited respectively to approximately 0.8 .mu.m and approximately 0.5 .mu.m. Therefore, it is not always possible to form an IDT having the very small line width mentioned above. Even if such an IDT having the very small line width mentioned above could be formed in the future, a driver for generating a high frequency of approximately 3.43 GHz cannot always be manufactured or can only be done at a very high cost. Also, it is not always possible to apply a high voltage to such an IDT. Further, in the case where the frequency of the surface acoustic wave is increased as mentioned above, the wavelength of the surface acoustic wave naturally becomes short, and therefore the surface acoustic wave is readily absorbed by the optical waveguide and the diffraction efficiency deteriorates.
On the other hand, an optical deflecting apparatus wherein, instead of operating a plurality of IDTs through switching as mentioned above, a single IDT is constituted as a curved-finger chirped IDT in which the transducer finger line width is changed continuously and the respective transducer fingers are in a circular arc shape, and the frequency of the surface acoustic wave and the direction of advance thereof are changed continuously over a wide range by the single IDT is disclosed in IEEE Transactions on Circuits and Systems, Vol. CAS- 26, No. 12, p. 1072, "Guided-Wave Acoustooptic Bragg Modulators for Wide-Band Integrated Optic Communications and Signal Processing" by C. S. Tsai. With the disclosed configuration, though the drawback with regard to fluctuations of the optical amount of the optical wave in accordance with the deflection angle can be eliminated, the frequency of the surface acoustic wave must still be adjusted to be very high, and therefore the same problems as mentioned above occur.
Accordingly, the applicant proposed in Japanese Patent Application No. 61(1986)-283646 an optical deflecting apparatus wherein no fluctuations in the optical amount of the optical wave as mentioned above are caused, and a wide deflection angle range is obtained even though the frequency of the surface acoustic wave is not adjusted to be very high. The proposed optical deflecting apparatus comprises:
(i) an optical waveguide formed of a material allowing propagation of surface acoustic waves therethrough, PA1 (ii) a first surface acoustic wave generating means for generating a first surface acoustic wave, which advances in a direction intersecting an optical path of a guided optical wave advancing inside of said optical waveguide and diffracts and deflects said guided optical wave, in said optical waveguide, and PA1 (iii) a second surface acoustic wave generating means for generating a second surface acoustic wave, which advances in a direction intersecting the optical path of said guided and diffracted optical wave and diffracts and deflects said guided and diffracted optical wave in a direction that amplifies the deflection caused by said diffraction, in said optical waveguide, PA1 (i) an optical waveguide formed of a material allowing propagation of surface acoustic waves therethrough, PA1 (ii) a first surface acoustic wave generating means for generating a first surface acoustic wave, which advances in a direction intersecting an optical path of a guided optical wave advancing inside of said optical waveguide and diffracts and deflects said guided optical wave, in said optical waveguide, and PA1 (iii) a second surface acoustic wave generating means for generating a second surface acoustic wave, which advances in a direction intersecting the optical path of said guided and diffracted optical wave and diffracts and deflects said guided and diffracted optical wave in a direction that amplifies the deflection caused by said diffraction, in said optical waveguide, PA1 (i) an optical waveguide formed of a material allowing propagation of surface acoustic waves therethrough, PA1 (ii) a first surface acoustic wave generating means for generating a first surface acoustic wave, which advances in a direction intersecting an optical path of a first guided optical wave advancing inside of said optical waveguide and diffracts and deflects said first guided optical wave, in said optical waveguide, and PA1 (iii) a second surface acoustic wave generating means for generating a second surface acoustic wave, which advances in a direction intersecting an optical path of a second guided optical wave advancing inside of said optical waveguide and diffracts and deflects said second guided optical wave, in said optical waveguide, PA1 (i) an optical waveguide formed of a material allowing propagation of surface acoustic waves therethrough, PA1 (ii) a first surface acoustic wave generating means for generating a first surface acoustic wave, which advances in a direction intersecting an optical path of a first guided optical wave advancing inside of said optical waveguide and diffracts and deflects said first guided optical wave, in said optical waveguide, PA1 (iii) a second surface acoustic wave generating means for generating a second surface acoustic wave, which advances in a direction intersecting an optical path of a second guided optical wave advancing inside of said optical waveguide and diffracts and deflects said second guided optical wave, in said optical waveguide, PA1 (iv) a third surface acoustic wave generating means for generating a third surface acoustic wave, which advances in a direction intersecting the optical path of said first guided optical wave diffracted by said first surface acoustic wave and diffracts and deflects said first guided and diffracted optical wave in a direction that amplifies the deflection caused by said diffraction, in said optical waveguide, and PA1 (v) a fourth surface acoustic wave generating means for generating a fourth surface acoustic wave, which advances in a direction intersecting the optical path of said second guided optical wave diffracted by said second surface acoustic wave and diffracts and deflects said second guided and diffracted optical wave in a direction that amplifies the deflection caused by said diffraction, in said optical waveguide,
wherein said first surface acoustic wave generating means and said second surface acoustic wave generating means are formed to continuously change the frequencies of said first surface acoustic wave and said second surface acoustic wave and the directions of advance thereof while satisfying the conditions of EQU k1+ K1= k2, and EQU k2+ K2= k3 PA2 wherein said first surface acoustic wave generating means and said second surface acoustic wave generating means are formed to continuously change the frequencies of said first surface acoustic wave and said second surface acoustic wave and the directions of advance thereof while satisfying the conditions of EQU k1+ K1= k2, and EQU k2+ K2= k3 PA2 said optical waveguides of said optical deflectors being disposed so that the optical waves radiated out of said optical waveguides scan in a line with respect to each other on a predetermined surface, and scanning extremities of the adjacent radiated optical waves adjoin each other. PA2 wherein said first surface acoustic wave generating means and said second surface acoustic wave generating means are disposed so that said first guided optical wave and said second guided optical wave radiated out of said optical waveguide scan in a line with respect to each other on a predetermined surface, and a scanning extremity of said first guided and radiated optical wave adjoin a scanning extremity of said second guided and radiated optical wave. PA2 wherein said first surface acoustic wave generating means and said third surface acoustic wave generating means are formed to continuously change the frequencies of said first surface acoustic wave and said third surface acoustic wave and the directions of advance thereof while satisfying the conditions of EQU k1+ K1= k2, and EQU k2+ K2= k3 PA2 said second surface acoustic wave generating means and said fourth surface acoustic wave generating means are formed to continuously change the frequencies of said second surface acoustic wave and said fourth surface acoustic wave and the directions of advance thereof while satisfying the conditions of EQU k4+ K3= k5, and EQU k5+ K4= k6 PA2 said first surface acoustic wave generating means, said second surface acoustic wave generating means, said third surface acoustic wave generating means, and said fourth surface acoustic wave generating means are disposed so that said first guided optical wave and said second guided optical wave radiated out of said optical waveguide scan in a line with respect to each other on a predetermined surface, and a scanning extremity of said first guided and radiated optical wave adjoin a scanning extremity of said second guided and radiated optical wave.
wherein k1 and k2 respectively denote wave vectors of the guided optical wave before and after being diffracted by said first surface acoustic wave, k3 denotes a wave vector of the guided optical wave after being diffracted by said second surface acoustic wave, and K1 and K2 respectively denote wave vectors of said first surface acoustic wave and said second surface acoustic wave.
The first surface acoustic wave generating means and the second surface acoustic wave generating means may each be constituted by, for example, a combination of a tilted-finger chirped IDT, in which the transducer finger intervals are changed stepwise and the directions of the transducer fingers are changed stepwise, with a driver for applying an alternating voltage the frequency of which changes continuously to the tilted-finger chirped IDT.
With the aforesaid configuration wherein the guided optical wave deflected by the first surface acoustic wave is again deflected by the second surface acoustic wave, a wide deflection angle range can be obtained as a whole even though the frequency bands of the first surface acoustic wave and the second surface acoustic wave are not adjusted to be so wide.